def find_substring(string, sub_string):
    length_string = len(string)
    length_sub = len(sub_string)
    if length_sub == 0:
        return 0
    if length_sub > length_string:
        return -1
    for i in range(length_string-length_sub+1):
        match = True
        for j in range(length_sub):
            if string[i+j] != sub_string[j]:
                match = False
                break
        if match == True:
            return i
    return  -1
print(find_substring("hello", "hel"))
print(find_substring("hello", "llo"))
